Oxidized carbon fibers

ABSTRACT

CARBON FIBERS, SURFACE OXIDIZED AFTER FINAL CARBONIZATION BY CONTACT WITH AN ABOUT 6 N AQUEOUS OXYGEN-CONTAINING INORGANIC ACID SOLUTION CONTAINING A SOLUBLE CHLORATE SALT, IMPROVES FIBER-RESIN INTERFACIAL BOND IN COMPOSITES THEREOF.

United States Patent 3,746,560 OXIDIZED CARBON FIBERS John C. Goan and Louis A. Joo, Johnson City, Tenn., assignors to Great Lakes Carbon Corporation, New York, NY. No Drawing. Filed Mar. 25, 1971, Ser. No. 128,166 Int. Cl. C08h 17/08, 17/10 US. Cl. 106-307 2 Claims ABSTRACT OF THE DISCLOSURE Carbon fibers, surface oxidized after final carbonization by contact with an about 6 N aqueous oxygen-containing inorganic acid solution containing a soluble chlorate salt, improves fiber-resin interfacial bond in composites thereof.

BACKGROUND OF THE INVENTION The invention herein described was made in the course of or under a contract or substract thereunder with the Air Force Materials Laboratory.

Composite materials, for use in the aerospace industry, are well-known to the art. Such materials comprise a resinous binder, as for example a polymerized epoxide and a filler, as for example asbestos, glass fibers, or carbon fibers.

Of the above named fillers, carbon fibers have received attention due to their high corrosion and temperature resistance, low density, high tensile strength and high modulus of elasticity.

Uses for such carbon-fiber reinforced composites include aerospace structure components, rocket motor casings, deep submergence vehicles, and ablative materials for heat shields on re-entry vehicles.

The incorporation of carbon or graphite particles in resin bases in amounts of up to 60 percent by volume will impart a heat-conducting property but not an electrical conductivity to the composite. Litant, in US. 3,406,126, teaches the addition of carbon yarn in as little as 0.05 percent by volume to the resinous matrix to impart electrical conductivity to the resulting composite. Such composites can be prepared from polyesters, polyvinyl chloride, polyepoxides, or like resins, and carbonized rayon, acrylic, or like fibers.

The primary structural properties of fiber-resin composites improve as carbon fiber content is increased from about 10 to about 65 volume percent, then decrease as the fiber content exceeds that figure. The preferred range of carbon fiber content is about 45 to 65 volume percent of fiber content is about 45 to 65 volume percent of fiber in the fabricated composite.

High modulus composites usually have low shear strengths parallel to the direction of the fibers of about 3000 to 4000 p.s.i. These low shear strengths are probably due to poor bonding between the carbon fibers and the matrix. Attempts to improve this bonding, particularly between rayon-based carbon fiber fillers and an epoxy resin matrix have been partially successful, but have resulted in a degradation of the ultimate tensile strength of the fiber and also of the fabricated composite.

Improved bonding has been accomplished by plating the fiber with various metals, as for example tantalum, with metal carbides, as for example whiskers of silicon carbide, and with nitrides.

More recently, carbon fibers have been treated with various oxidizing agents in order to etch the surface of the fiber, J. W. Johnson in Belgian Pat. 8,995/ 68. Among these methods for cathodic oxidation, ionic bombardment, heat treatment in oxygen, ozone, or air, thermal treatment in carbon dioxide, immersion in molten potassium nitrate, concentrated nitric acid or concentrate sulfuric acid, elec- 3,746,560v Patented July 17, 1973 trolysis, chromic-orthophosphoric acid oxidation, perchloric acid oxidation, and the like. In most cases the oxidative treatment of rayon-based carbon fibers resulted in a decrease in ultimate tensile strength of the fiber and of the fiber-resin composite.

SUMMARY OF THE INVENTION By the process of this invention, carbon fibers of acrylic origin are surface oxidized after carbonization by contacting the fibers with an about 2 N to about 6 N aqueous solution of an oxygen-containing mineral acid containing from about 5 to about 20 weight percent sodium chlorate or equivalent chlorate ion concentration derived from a soluble chlorate salt for up to about 30 minutes at the refiuxing temperature of the solution.

DETAILED DESCRIPTION OF THE INVENTION High modulus acrylic-based carbon fibers useful for this invention are defined as those fibers possessing a tensile strength of greater than 10 p.s.i. and a Youngs modulus greater than 20 l0 p.s.i. Such fibers can be prepared by the method of Shindo, Studies in Graphite Fiber Report No. 317 of the Government Research Industrial Institute, Osaka, Japan, 1961, and Tsunoda, US. 3,285,868. Typically, acrylic-fibers can be stretched to about 50 to 100 percent or more of their original length while heating in the presence of water or steam to about 100 C., oxidized in an oxidizing atmosphere at about 200 to 300 C. for a period of up to 4 hours, oxidized in a second stage in an oxidative atmosphere at 200 to 375 C. and pyrolyzed and/or graphitized at 1000 to 3000 C. in a non-oxidizing atmosphere to prepare a carbon fiber possessing a high modulus of elasticity and a high tensile strength.

Fibers prepared by the above disclosed method can be treated by the process of this invention to prepare composites of superior shear strength.

In carrying out the process of this invention, carbon fibers prepared by the above-described or similar method are contacted with an about 2 to about 6 Normal aqueous solution of a substantially fully inonizable inorganic oxy-acid and containing about 5 to about 20 weight percent of sodium chlorate or an equivalent chlorate ion concentration derivable from chloric acid or from any water soluble salt thereof.

The inorganic oxy-acids useful for the method of this invention include sulfuric acid, phosphoric acid, nitric acid, and the like.

Soluble salts of chloric acid suitable for use in the process of this invention include the potassium salt, lithium salt, calcium salt, ammonium salt, and the like in addition to the above-mentioned sodium salt.

In order to increase the effective reaction rate between the fiber surface and the oxidizing solution, the solution is maintained at about the boiling point thereof while the fiber is being contacted therewith. At this temperature, a

.contact time up to about 30 minutes is sufficient to cause a demonstrable increase in the shear strength of the fiberresin composites manufactured therefrom with little or no degradative effect onother structural properties of the fiber or the composites. It is obvious to one skilled in the art that were the reaction to be carried out at lower temperatures, longer reaction time would be required to cause the same effective reaction.

In a preferred embodiment, the fiber is contacted with a 6 N aqueous solution of sulfuric acid containing about 10 to about 15 weight percent of sodium chlorate for about 8 to about 12 minutes at the boiling point of the solution.

After reaction, the fibers are washed well with water and air dried.

to give a high shear strength composite. Exemplification of this method has been provided by Rees, U.S. 3,276,931, and Warner, U.S. 3,281,300.

The physical properties if the prepared composite including tensile, compression, and shear strengths, are measured by methods also well known in the art. More specifically, in order to prepare test composites, the fiber is wound onto a 7 inch diameter drum which contains an exterior 0.005 inch Teflon sheet coating. A transverse guide driven at a constant rate based on yarn diameter to provide parallel alignment of the yarn without voids or overlap of the fibers. While winding, a solution of 38 weight percent epoxy resin (Shell Epon 826) 12 weight percent meta-phenylenediamine (E. I. du Pont de Nemonrs), and weight 50 percent anhydrous methyl ethyl ketone in an amount 22 /2 times that required for the composite is added to the winding and the mandrel is heated to provide a surface temperature of 75 C. in order to effect a precure or B-stage in the resin system and evaporate the excess solvent. The additional material is provided to permit adequate flow and bleed-out. Winding is continued until a single layer of composite has been accumulated on the drum. The resulting composite is cut transversely, pulled from the drum, and spread flat on Teflon sheeting to provide a B-stage tape. Such tape is cut into appropriately dimensioned segments and the segments are stacked into a Teflon-lined mold, aligning the fibers, until an amount needed to form a 0.12 inch thick composite bar has been accumulated. The mold containing the stacked tapes is placed in a heated-platen press, held under a pressure of millimeters of mercury for one hour, then heated at 100 C. for 2 hours under a pressure of 300 p.s.i.g. and at 200 C. for one hour under the same pressure to effect cure.

The cured composite is tested for fiexural strength, flexural modulus, tensile strength, tensile modulus, volume percent of fiber, shear strength, and density. A sample composite bar is loaded in a three-point configuration on a 2 inch span (the supports and loading surfaces being the radial faces of 0.5 inch diameter steel pins). Stress is applied until failure, giving a linear stress-strain curve from which the flexural strength and flexural modulus of the composite can be calculated. A second sample of the composite is loaded in a three-point configuration on 0.4 inch centers consisting of the radial surfaces of 0.375 inch diameter steel pins, providing a length to depth ratio of 3.3: 1. The bar is flexed to failure. Depending upon the tensile properties of the reinforcing yarn and the quality of the resin matrix to graphite yarn interfacial bonding, three predominate modes of failure are noted. A transverse (tensile) failure showing a sharp peak in the stress-strain curve at the failure point results from high shear properties in conjunction with relatively lower tensile strength properties of the yarn. Shear strength values obtained with transverse failure of this type are not true indications of interlaminar shear strength but are minimum values since the tensile strength of the bar failed before a true shear failure is attained.

Low shear strength bars failed at the carbon-resin interface parallel to the long dimension of the sample.

Complex failures show characteristics of both of the above failure types.

What is claimed is:

1. A method of treating carbon fibers which comprises contacting the fiber with an aqueous solution containing about 5 to about 20 percent by weight of sodium chlorate or an equivalent concentration of chlorate ion and about 2 to about 6 Normal substantially fully ionizable inorganic oxy-acid at the refluxing temperature of the solution at normal sea level atmospheric pressure for about 8 to about 12 minutes.

2. A method of claim 1 which comprises contacting the fiber with an aqueous solution containing about 10 to about 15 percent by Weight of sodium chlorate or an equivalent concentration of chlorate ion and about 2 to 6 Normal substantially fully ionizable inorganic oxy-acid at the refluxing temperature of the solution at normal sea level atmospheric pressure for about 8 to about 12. minutes.

References Cited UNITED STATES PATENTS 3,476,703 11/1969 Wadsworth l06-307 3,330,799 7/1967 Voet 106-307 OTHER REFERENCES Miyamichi et al., Chem. Abstracts, vol. .64, 1966, col. 12,862(b)(c).

Sach et al., Chem. Abstracts, vol. 71, 1969, col. l03,026(h).

JAMES E. POER, Primary Examiner U.S. Cl. X.R. 423-447 

